In this case, the durability refers to the loss of data/objects stored per year - if you're sending multiple PBs of data off to Glacier, you want to be able to retrieve them many years later. Even 5 9s would mean that 1 object out of 100,000 is lost every year, which is quite poor.
Those claims are from glacier: http://aws.amazon.com/glacier/faqs/ which is priced the same per GB of data.
In this case, the durability refers to the loss of data/objects stored per year - if you're sending multiple PBs of data off to Glacier, you want to be able to retrieve them many years later. Even 5 9s would mean that 1 object out of 100,000 is lost every year, which is quite poor.